Full Length ArticleOne-step synthesis of Fe-doped surface-alkalinized g-C3N4 and their improved visible-light photocatalytic performance
Graphical abstract
The interlayer of Fe species on a semiconductor can act as a cocatalyst for photogenerated electrons extraction and offering active sites to boost the ROSs yield, as well as speed up the reaction process.
Introduction
Antibiotics are the world’s largest and most used drugs, which play an essential role in the prevention and treatment of diseases [1], [2], [3]. With the increasing use of antibiotics, a numerous of antibiotics have been released into the environment by pharmaceutical wastewater and aquaculture wastewater year by year [2], [3], [4]. Tetracycline (TC), as a typical antibiotic, has been widely used in a variety of human, veterinary and farming application [5]. TC give rise to a serious threat that destroys the ecological environment balance and human health [3], [4], [6]. Many conventional processing techniques have been adopted to solve this problem, such as electrolysis [7], adsorption [8] and microbial degradation [9] etc. Among these ways, developing a green and efficient technology to decompose the TC from wastewater is extremely necessary.
Up to now, to better utilize visible-light, many semiconductors are used as photocatalysts for degradation of antibiotics, such as Graphitic carbon nitride (g-C3N4) [10], [11], [12], [13], [14], [15], TiO2 [16], [17], [18], ZnO [19], [20], BiVO4 [21], [22], and CoO [11] etc. Among these semiconductors, g-C3N4, as an n-type semiconductor, displays interesting electronic band structure, physicochemical stability [23], [24], [25]. However, the photocatalytic activity of bare g-C3N4 is confined within some drawbacks, which include poor light absorption above 460 nm, the high recombination rate of e− – h+ pairs, and lack of conductivity [26], [27]. For the sake of enhancing the photocatalytic performance of bulk g-C3N4, various methods have been attempted such as doping and nanostructure design [28], [29], copolymerization [30], [31], the formation of g-C3N4 based heterojunction photocatalysts with other suitable semiconductors [11], [32], [33]. Among these strategies, doping (metal or non-metal) has been extensively used as a valid method to modulate energy gap of semiconductors for the treatment of conductive, optical, or other physical properties [34], [35]. Doping Fe has been recognized as a facile and efficient approach to amend g-C3N4 [24]. More importantly, g-C3N4 is rich in N atoms, which are filed with six lone pair electrons [36]. This unique unit structure is quite appropriate for Fe inclusion. Previous reports indicate that the introduction of a certain amount of Fe species strongly altered the electronic properties of g-C3N4, which exhibited excellent photocatalytic performance with the addition of H2O2 [36]. Fang et al. constructed a novel ternary Fe/graphene/g-C3N4 composite photocatalyst and realized the degradation of a pollutant from water [37]. Tonda et al. designed a Fe doped g-C3N4 nanosheets could be obtained by the method of ultrasonication-assisted liquid exfoliation and degraded water pollutant under natural sunlight [38]. However, Fe modified bulk g-C3N4 still undergo the relatively slow reaction kinetics. The structure of pure g-C3N4 is similar to graphene, the synthesis of K-modified g-C3N4 [39] can effectively enhance the separation rate of photo-generated carriers. In the meantime, surface-alkalinized g-C3N4 [40], [41] can further trap the h+ to reduce the recombination of e+/h+ pairs [42]. Therefore, we designed the catalyst of Fe-doped surface-alkalinized g-C3N4 for the aim of enhancing degradation ability.
Herein, we designed a one-step method of calcining to synthesize Fe-doped surface-alkalinized g-C3N4 by using melamine as precursors. The morphology and composition, structure, and optical properties of the resultant samples were characterized by scanning electron microscopy (SEM) and TEM-mapping, Fourier transform infrared spectra (FT-IR), X-ray photoelectron spectroscopy (XPS), UV/Vis diffuse reflectance spectra (DRS) and photoluminescence (PL). The photocatalytic performance and stability were evaluated by the degradation of tetracycline (TC) under visible-light irradiation. Additionally, a possible photocatalytic mechanism was proposed based on the results of electron spin resonance (ESR) and the trapping experiments of active species.
Section snippets
Photocatalysts preparation
All chemical reagents were purchased and directly used without further treatment. The bulk g-C3N4 powders were prepared by a facile thermal treatment of melamine based on the previous paper [41]. Typically, melamine was calcined with a covered crucible in a tube furnace at a heated rate of 2.5 °C/min and held for 4 h at 550 °C, and kept in inert gases. The sample obtained was named as CN.
To fabricate the potassium (K) embed and hydroxyl (OH) grafted g-C3N4, typically, melamine (1.5 g) and KCl
Characterization of as-prepared samples
The phases of all the samples were characterized by XRD analysis. Fig. 1a shows the XRD patterns of pure CN, and CN/K, CN/K/OH, CN/K/OH/Fe composites. The two characteristic peaks of the CN sample displays at 13.22° and 27.70°, indexing to the (1 0 0) and (0 0 2) diffraction planes of pure g-C3N4, respectively. Compared with the peaks of bulk g-C3N4 appears at 13.0° and 27.4° [43], which shows a decrease in the (1 0 0) and (0 0 2) planes after boiling water washing. After the introduction of
Conclusions
In conclusion, the Fe-doped surface-alkalinized g-C3N4 photocatalysts were successfully synthesized via the one-step calcination method. The photocatalytic performance of CN could be greatly improved by doping Fe on the surface-alkalinized of g-C3N4. Doping Fe plays a key role in determining the photocatalytic activity of CN/K/OH/Fe. The 0.05 wt% Fe modified CN/K/OH displays the highest photocatalytic efficiency under visible-light irradiation. In addition, the as-synthesized materials
Acknowledgements
This work is financially supported by the National Natural Science Foundation of China (No. 21506079, 21777063, 21476098). Natural Science Foundation of Jiangsu Province (BK20161363)
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